This invention relates to a laser, and more particularly to a hollow cathode type laser.
In a "coaxial hollow cathode" type discharge device, an anode is disposed outside the cathode cylinder and electrons flow through slots cut in the cathode cylinder toward the anode, as described by W. K. Schuebel in IEEE Journal of Quantum Electronics, Vol. QE-6, pp. 574-575, 1969. This type of hollow cathode discharge device is most popular because it advantageously provides a uniform negative glow discharge in the laser active medium along the axis of the cathode cylinder. However, it has drawbacks of its own. First, it exhibits constant voltage-current characteristics. Secondly, it sometimes provides an unstable discharge which will possibly result in arc discharge. Thirdly, its cathode cylinder is heated too much because no effective measures are taken to cool the cylinder. Overheating of the cylinder shortens the life of the laser and indirectly makes it difficult to achieve an adequate distribution of metal vapor pressure inside the cathode cylinder.
Another discharge device called "multiple anode hollow cathode discharge device " is known, as disclosed by J. A. Piper and C. E. Webb in Journal of Physics D: Applied Physics, Vol. 6, pp. 400-407, 1973. This device uses many pin-shaped anodes, and electrons produced by negative glow discharge flow through holes made in the cathode cylinder toward the pin-shaped anodes. Indeed the device effects a stable discharge and its cathode cylinder can be easily cooled. But the discharge is not sufficiently homogeneous along the axis of the cathode cylinder.
Still another discharge device called "hollow anode cathode discharge device" is known, as proposed by K. Rosza, M. Janossy and L. Csillaq in Physics Letters, Vol. 63A, pp. 231-232, 1977. This device achieves an extremely stable discharge which is very homogeneous along the axis of the cathode cylinder. Further, the cathode cylinder, which is the outermost member, can easily be cooled. This device is disadvantageous, nonetheless. It exhibits somewhat linear voltage-current characteristics; a rapid voltage rise is inevitable with an increase of current. This means that the temperature of electrons sharply rises as more and more current is applied. The device cannot therefore be suitable for use in lasers whose upper level is low, such as He-Cd lasers or He-Zn lasers. It may be used only in rare gas ion lasers. The device is disadvantageous also in that it is rather difficult to accomplish a negative glow discharge of a small radius within the anode arranged coaxially with the cathode cylinder.